Pads are going to vibrate, ... Period. When that vibration transfers to the calipers, they act like amplifiers and cause the squeal we can hear. There are two ways of attacking that vibration. One is to use a glue-type product to bond the backing plates of the pads to the caliper and piston in an attempt to prevent the vibration. This method never works for long. As proof, when you remove old pads, you will never find any that are still stuck on solidly. Any remains of that glue must be removed when installing new pads. More on that later.
The second plan of attack is to allow the vibration to occur but to isolate the pads from the calipers and pistons. One method used a lot on imports was to use steel or thick paper shims on the pad backing plates. Thick paper shims are stuck on with sticky material like a Post-it note. When removed, you will find them bunched up inside the piston and between the two fingers of the caliper that hold the outer pad. That is proof the pad has been moving around but they have been fairly successful at reducing squeal. Steel shims can't bunch up. Some are stuck on with the sticky material that only has to hold them in place until they are in the installed position. Other designs use little tabs as clips to hold them in place. Those steel shims have been known to be overlooked when changing pads. When the pad is removed, you think that's it and don't notice the shim stuck or rusted to the piston or caliper. If you're installing a new rotor or the old one isn't worn much, there is very little extra room for the new pads. One extra steel shim can be too much for that space. If the old rotor is worn down a little, you may get the caliper installed with an extra shim. That extra shim won't hurt anything as far as braking power is concerned, but if it makes that caliper tight, that brake will drag until the new linings wear down a little.
Some steel shims are riveted to the pad backing plates. No grease is needed underneath them between the backing plate and shim, but the shim is still greased where it contacts the piston and caliper. It is somewhat common for the rivets to rust away and the shims break free. Those are the ones that are easy to overlook when installing new pads.
Today shims have pretty much gone away in favor of other improvements. Higher quality lining material has a higher tendency to squeal. Some of the things professionals will do to reduce squealing include beveling the leading edges of the pads and lubing key points. Think of the leading edge, (the first part of the lining the rotor sees as it rotates around to the pad), as dragging your fingernails down a black board. Removing that sharp edge eliminates that source of a squeal. We used to grind about 1/8" of the edges off on a bench grinder but that is excessive and it eliminates the squeegee action after driving through deep water. That can lead to brake fade. GM still supplies pads with almost an inch of material removed in an attempt to stop brake squeal. What I have found is it is sufficient to simply break the edge on new pads. Dragging them over a concrete floor for a foot or two is enough to stop that source of squeal. A couple of passes with a flat file works well too. All you want to do is remove that sharp edge. It seems if the pads don't squeal during the break-in period, they won't squeal later either.
Before installing the new pads, run a flat file over the rim of the piston and the outer fingers of the caliper. The goal is not to shine them up; it is simply to remove any high spots of dirt, rust, or that glue I mentioned earlier. You want the pads to sit flat on those surfaces. If you're reinstalling old pads, remove any dirt or rust from the backing plates with a wire wheel on a bench grinder, again so they sit flat.
It is also important to lubricate a lot of points. High-temperature brake greases are made just for this purpose. "Rusty Lube" is one trade name I'm very familiar with. The key ingredient is molybdenum disulphide. There should be grease on the pad backing plates where they contact the piston and caliper fingers. Since you can't stop the pads from vibrating, that grease will let them do so freely without transferring the vibration to the caliper where it would be amplified. Whatever the caliper mounts on should also be lubed. That can be a pair of slide bolts, cast iron slides built into the mounts on the steering knuckle, or both. Most cars have single-piston calipers. The piston pushes on the inner pad, and that action pushes the caliper inward on its mount to apply pressure on the outer pad. If those mounts aren't lubed, and the caliper can't slide freely, the outer pad will have little tendency to apply under light braking. That will lead to accelerated wear of the inner pad compared to the outer pad. Ford had a huge problem with their truck brakes with sticking caliper slides. Logic would tell you the design will cause the calipers to stick on their mounts AFTER the brakes were released. That led to rapid wear of the outer pads since that pad stayed applied all the time. 15,000 to 20,000 miles was typical life of the front pads, and it got even worse if the mounts were rusty or impacted with mud. For vehicles with slide mounts, the flat file should be used on them to remove any dirt or rust so the caliper will move freely.
Some calipers use two pistons side-by-side on the inside. For general brake work, they are treated the same as single-piston calipers. Some calipers have a second piston on the outside pad. Those calipers usually mount solidly to the knuckle and don't have slides or mounts that require lubrication.
While on the topic of grease, one place that is often overlooked is on vehicles with rotors that slide off the hub, (not held on with wheel bearings and a nut). These are the rotors that flop around until the wheel is installed to hold them tight. A light coating of grease should be placed around the hub where the center hole of the rotor contacts it. There is a microscopic amount of movement there when cornering and that can set up a crunching noise. GM front-wheel-drive cars had a big problem with that in the '80s and early '90s, but it can happen to any car. That can be a real hard problem to find if you're not aware of it. Any grease works in that location but a high-temperature grease is best to prevent smoking.
While we're on the subject of squeals, one thing you can do to CAUSE a squeal is to get grease on the linings or rotor friction surface. One local shop owner makes his people throw new pads away if they get grease on them, but it is acceptable to use them as long as any grease is washed off with brake parts cleaner before they get hot. Once hot from normal driving, the grease will soak into the linings, cause a squeal, and will be impossible to remove. Even fingerprint grease must be washed off. You will also see professionals carry a rotor by grabbing it anywhere except on the friction surface.
If your brakes are still not releasing, start by opening a bleeder screw on the sticking caliper to see if it releases. If it does not release, there is a mechanical issue that must be determined. If the bleeder screw is rusted tight and you're afraid of twisting it off, you can crack the hose connection too, but not so much as to let air in that you won't be able to remove. If the caliper is still sticking, besides the extra shim, look for a tab on a metal shim that got bent over and is between the shim and pad backing plate. That will add thickness that the piston can't retract enough to accommodate, but usually the shim will just bend a little and you won't even notice it. Some inner pads have spring-metal clips to hold them to the piston. Check for one of the two or three fingers that got bent and is between the piston and pad backing plate. Some imports use two small steel pins that go through the calipers and holes in the top of the pads to hold the pads in position. Two anti-rattle wires are used between those pins to hold the pads away from the rotors. If one of those wires is deformed, it can rub against the rotor but it shouldn't cause the brake to stick. Another design uses a flat spring-steel plate between the pads. That plate is clipped on over the access hole in the middle of the caliper. Check for bends or kinks in that plate that allow it to rub on the outer edge of the rotor. That should be obvious when you spin that wheel by hand, but it might not make noise with the wheel removed. That's because the rotor can move on the hub enough to move away from that plate.
Wednesday, October 13th, 2010 AT 1:50 PM